1. Field of the Invention
The present invention relates in general to a delivery control device for variable displacement hydraulic pumps for negative type delivery controlling hydraulic systems and, more particularly, to a structural improvement in such delivery control devices for instantly increasing the pump delivery in response to starting the actuator, the invention also relates to hydraulic systems having such delivery control devices.
2. Description of the Prior Art
"Negative type controls for hydraulic pump delivery" generally means that the delivery of the hydraulic pump is controlled in such a manner that the delivery can be reduced by regulating a swash plate or by increasing the inclination angle of the swash plate using a given pilot pressure under the condition that the hydraulic pump is preset to output the maximum pump delivery in the initial state of the pump.
With reference to FIG. 1, there is shown in a block diagram a typical hydraulic system whose pump delivery is controlled using the negative type. As shown in the drawing, the system includes a plurality of directional control valves 1, 2 and 3, which are connected to a variable displacement hydraulic pump P through a common center bypass line 11 in order for letting a plurality of actuators A, B and C be operated by the pressurized fluid of pump P. In the above hydraulic system, the pump delivery or the pressurized fluid of pump P is drained to a return tank T by way of an orifice 12 when all control valves 1, 2 and 3 are in their neutral positions, or when neither of the actuators A, B and C are operated. The center bypass line 11, after passing the control valves 1, 2 and 3, is branched to a pilot line 13, which is used for supplying pilot pressure Pi to pump P to regulate the inclination angle of the swash plate of pump P. In the above hydraulic system, pump P is preset to output maximum pump delivery in the initial state of pump P. When the pilot pressure Pi acts on the swash plate of pump P, the inclination angle of the swash plate is regulated or increased in proportion to the pilot pressure Pi so that the delivery of pump P is reduced. In the above block diagram, reference symbol Qp denotes a pressurized fluid flow or the delivery of pump P measured at the point "a" of the bypass line 11, while the reference symbol Qn denotes a pressurized fluid flow, after passing control valves 1, 2 and 3, measured at point "b" of line 11.
The relation between the pressure Pi and the fluid flow Qn and the relation between the pressure Pi and the fluid flow Qp in the above hydraulic system are represented by the graphs of FIGS. 2A and 2B respectively.
As represented in the graphs of FIGS. 2A and 2B, the pilot pressure Pi is in proportion to the fluid flow Qn and this makes the fluid flow Qp be in inverse proportion to the pilot pressure Pi. Otherwise stated, when all of the control valves 1, 2 and 3 are in their neutral positions, or when neither of the actuators A, B and C are being operated, the fluid flow Qn is maximized, that is, Qn=Qp. In this case, the pilot pressure Pi is increased due to the negative pressure generated by the pressurized fluid passing the orifice 12.
The inclination angle of the swash plate of the pump P thus varies by the increased pilot pressure Pi so that the pump delivery Qp is reduced. On the other hand, when at least one of the above actuators A, B and C is operated, the pump delivery Qp is partially applied to the operated actuator so that the fluid flow Qn becomes the fluid flow resulting from subtracting the fluid flow for the operated actuator from the pump delivery Qp. Such reduction of the fluid flow due to fluid consumption by the operated actuator makes the pilot pressure Pi be proportionally reduced so that pump delivery Qp is increased.
That is, the pump delivery of the above hydraulic system will be automatically reduced when there is no operated actuator. However, the pump delivery will be automatically increased when at least one actuator is operated.
However, the above hydraulic system has a problem that the pump delivery Qp is not instantly increased in response to the start of the actuator, but slowly increased after some retardation.
The above problem is noted to be caused by the fact that the system is not in an ON/OFF state, in which the ON/OFF state of valves 1, 2 and 3 are completely opened or closed, but in a transitional state, in which each control valve is partially opened or closed, due to the spool strokes of the valves 1, 2 and 3. As shown in FIG. 3, the transitional state of the valves 1, 2 and 3 means that the internal lines, that is, the bypass line 11, an actuator fluid supply line 14 and an actuator fluid return line 15, of the control valves form orifices respectively. When the actuators A, B and C are applied with a load (higher than the load applied thereto when the pressurized fluid of the pump P passes the orifice 12) during the above transitional state, the pump delivery Qp does not flow in the actuator fluid supply line 14 but totally flows in the bypass line 11 with a relatively lower load. Therefore, both the fluid flow Qn and the pilot pressure Pi are not even slightly increased irrespective of the start of the actuator and the pump delivery Qp is not increased at all. This means that the pump delivery characteristic of the typical hydraulic system is changed by the load acting on actuators A, B and C.
If briefly described, the typical hydraulic system whose pump delivery is controlled using the negative type has a problem that the pump delivery can not help being slowly increased after some retardation for achieving sufficient spool stroke of a control valve even though it is preferred to instantly increase the pump delivery in response to starting the actuator.